To better understand the antigenic nature in the immune response to P. carinii, we have undertaken to purify the major surface antigen of rat and human P. carinii. It is necessary to use P. carinii from both sources because antigenically they are different and specifically the major surface antigen in rat and human appear to be different. The major surface antigens of both rat and human P. carinii have been purified by treatment of intact organisms with liticase followed by separation on a sizing column and subsequent separation by ion-exchange chromotography. Using the above protocol, we have purified both rat and human P. carinii antigens in sufficient quantities to do preliminary biochemical analysis. Using cross- linking studies, we have documented that although the molecular weight of the antigen is about 95-100,000 on SDS-PAGE, in its natural form it appears to have a molecular weight of about 280,000-300,000. The antigen is a glycoprotein with about 10% of the composition of the protein being accounted for by carbohydrates. The specific carbohydrates have been partially characterized. We are currently in the process of attempting to obtain an amino acid sequence on the protein with the hopes of ultimately identifying the gene for this protein and subsequently expressing it in large enough quantities to be able to do a variety of studies aimed at investigating the epidemiology in immunologic response to P. carinii. In addition, the antigen from human P. carinii has been used to develop an Alisa to look at seriologic response to human P. carinii. These studies are currently underway to evaluate a large number of human sera for reactivity with this antigen. These studies are important for developing new diagnostic tests for pneumocystis.